The invention relates to inspection of dielectric materials used in the electronics industry such as flux on its own or in solder paste.
Such materials are used in two main branches of the electronics industry, namely (a) surface mount technique (SMT) circuit production processes and (b) chip scale packaging (CSP).
The CSP branch includes ball grid array (BGA) and flip-chip processes. In BGA processes an array of solder balls is deposited on a substrate which was previously coated with flux. The flux, when heated during the reflow process, improves the cleanliness of the resulting solder joint as well as improving the mechanical and electrical connectivity between the solder and the pad to which it is soldered. In flip-chip packaging, a solder mask covers a copper substrate to reveal an array of spots at which the copper is exposed. Flux is applied over the mask and a flip-chip having an array of balls on a lower surface is deposited onto the substrate with the balls in registry with the mask spots. In both BGA and flip-chip processes the solder is reflowed afterwards to complete the package.
A further branch is now emerging, namely circuit production processes using adhesives. This avoids the need for use of solder and therefore flux is also not required. Ultraviolet (UV) light is used for curing the adhesive.
In the above processes, both chip-scale and circuit-scale, various techniques are used for application of a dielectric material such as flux, solder paste incorporating flux, or adhesives. These techniques include stencil printing, pin transfer, dispensing, dipping, and jetting. Whichever technique is used, there are very stringent requirements to be met to ensure good quality chip or circuit production, and the stringency arises particularly because of the ever-decreasing scales involved. The following summarises some of the more important requirements.
Tackxe2x80x94Apply enough flux to retain solder balls from 300-762 xcexcm in place during reflow.
Solderabilityxe2x80x94Apply enough flux to remove oxides on the solder balls and board pads.
Residuesxe2x80x94Limit residues by applying an appropriate amount of flux which will activate and burn off during reflow.
Barrier Effectxe2x80x94Apply flux such that a xe2x80x9cflux barrier effectxe2x80x9d which blocks the seating and soldering of solder balls or array components does not occur.
Contaminationxe2x80x94Apply a controlled amount of flux such that smears, contamination with the surrounding solder mask, and blockages in the stencil are avoided.
Process Capabilityxe2x80x94Apply a repeatable quantity of flux despite ambient variations.
Flux thickness is related to the number of missing balls at reflow and to occurrence of voids in paste. Voids in solder balls are in turn linked to reduction in joint reliability and they affect the joint high frequency signal propagation. Excess flux can also interact with solder mask to cause excessive residues and increase migration between adjacent solder joints. Defect rates at ball attach also have been shown to increase with increasing flux thickness. For flip chip soldering, a 20 micron minimum and 50 micron nominal flux thickness is recommended based on solder ball coplanarity specifications and the solder ball height.
Meeting quality standards is often complicated by various attributes of the dielectric material. For example, flux is a non-Newtonian liquid. Therefore, its viscosity changes with the speed with which it is worked, and also temperature and humidity. Also, in screen printing, there is a tendency for air bubbles to be worked into the flux.
Clearly, in such environments inspection of the substrate for volume and location of dielectric material is essential. However current inspection methods are often inadequate for measuring to the tolerance required. One inspection problem is that many commercially used solder fluxes are nearly transparent and so conventional machine vision techniques are unreliable.
In another approach, United States Patent Specification No. U.S. Pat. No. 5,820,697 (IBM) describes a method of joining metal surfaces in which there is post-reflow inspection of the solder connection for residual flux. The method involves mixing a water soluble fluorescent dye with a water soluble soldering flux to form a mixture. The metal surfaces are heated to a temperature at which the solder material softens and for a time period to form a solder connection. The solder connection is washed to remove the dye and the flux. The solder connection is then illuminated to cause the dye to fluoresce so that residual flux is detected. This approach may be effective in some situations. However, it suffers from the disadvantage of the requirement to add a fluorescent dye and so it is invasive. Choice of dye is difficult and requires great care because it must not degrade at the temperatures involved and must not have an adverse effect on the flux and circuit materials. Also, mixing of dye adds an additional process step which must be carefully controlled and care must be taken to ensure that the dye does not have an adverse effect on the other materials. Also, this process provides limited quality information, focused on indications of presence or absence of post-reflow flux residues.
One object is to provide an inspection method and system which provide more comprehensive information about deposition of dielectric materials in the electronics industry.
Another object is that the above is achieved in a non-invasive manner without addition of extra processing steps or addition of an extra material.
According to the invention, there is provided a method of inspecting a dielectric material deposited on a substrate in an electronics production process, the method comprising the steps of:
(a) directing radiation excitation at the material to cause radiation emission by inherent fluorescence of the material,
(b) detecting the emission and determining emission intensity data, and
(c) processing said emission intensity data to generate output data indicating volume of the material according to a relationship between said emission intensity and material volume.
In one embodiment, step (b) comprises determining intensity data across the material surface and step (c) comprises generating output data indicating a defect if non-uniformity above a pre-set level occurs over the material surface.
In another embodiment, said output data includes an indication of presence of a void within the material or an unacceptable material height non-uniformity.
In one embodiment, the excitation wavelength is in the range of 320 nm to 390 nm.
In one embodiment, the emission is detected after filtering out radiation outside of an emission wavelength range from a sensor field of view.
In a further embodiment, radiation having a wavelength below 420 nm is filtered out.
In one embodiment, the steps (a) and (b) are carried out simultaneously and there is activation of the excitation only during emission detection.
In one embodiment, the excitation is generated by switching LEDs.
In one embodiment, the duration of excitation is less than 100 ms.
In another embodiment, the duration of excitation is in the range 5 ms to 80 ms.
In one embodiment, the direction of excitation is at an angle of greater than 50xc2x0 from the sensing axis to minimise sensing of reflected unwanted radiation.
In one embodiment, the angle is between 55xc2x0 and 80xc2x0.
In one embodiment, the method comprises the further steps of directing visible radiation at the material, sensing reflected visible radiation, and using said sensed visible radiation to determine material position with respect to fiducials.
In one embodiment, said visible radiation is generated by near-on-axis LEDs with respect to the sensing axis.
In one embodiment, the dielectric material is solder flux.
In one embodiment, the method is carried out on flux before application of solder.
In a further embodiment, the method comprises the further step of providing in-line process control feedback to a flux dispensing station to avoid downstream processing defects.
In one embodiment, the material is flux deposited on a chip substrate in a BGA or flip chip production process.
In one embodiment, the material is flux deposited on a circuit board in a SMT production process.
In one embodiment, the material is a component adhesive.
According to another aspect, the invention provides an inspection system comprising:
a radiation source for directing radiation excitation at a wavelength to cause inherent fluorescent emission in a dielectric material,
a sensor for detecting inherent fluorescent emission, and
an image processor comprising means for determining emission intensity data from the detected emission and for generating output data indicating volume of the material according to a relationship between said emission intensity and material volume.
In one embodiment, the radiation source and the sensor are mounted on an inspection head of a robotic gantry system.
In one embodiment, the radiation source is mounted to direct the excitation at an angle of in excess of 50xc2x0 to the sensor axis.
In one embodiment, the radiation source comprises a ring of LEDs mounted for low-angle diffuse illumination of the material and the sensor is mounted for positioning vertically above the material.
In one embodiment, the system further comprises a filter for blocking non-emission radiation from the sensor.
In a further embodiment, the system further comprises a visible light source, a visible light sensor mounted to sense visible light reflected from the material, and means in the image processor for determining location of the material with respect to fiducials.
In one embodiment, the radiation source comprises means for emitting excitation at a wavelength in the range of 320 nm to 390 nm.
In one embodiment, the image processor is programmed with excitation intensity data derived from monitoring excitation and responsive emission from a standard fluorescent material.